This invention is directed to an ink supply system for ink jet printers and, in particular, to an ink supply system for supplying ink to the nozzle of an ink jet printer which can both monitor the consumption of ink and detect and trap air bubbles in the printer.
The ink-on-demand type ink jet printing apparatus which selectively projects ink droplets from a nozzle to effect printing on a recording medium by reducing the volume of a pressure chamber is advantageous in that such an apparatus can print on plain paper. Additionally, in ink jet printers, the printing noise is low and the apparatus requires low energy to operate. However, a major problem with ink jet printers is that it becomes difficult to project the droplets of ink if an air bubble or other obstruction is present in the pressure chamber which inhibits the flow of ink to the head or nozzle thereby interrupting the clear printing operation. Such air bubbles can form in the ink supply container and can flow into the print head. Also, air bubbles may form in the print head and flow back through the nozzle into the supply line. Thus, it becomes critical in ink jet printers to eliminate and prevent air bubbles from interrupting the printing operation.
One type of ink supply system for an ink jet printer which traps and releases air bubbles is described in U.S. Pat. No. 4,149,172 entitled INK SUPPLY SYSTEM FOR PIEZOELECTRICALLY OPERATED PRINTING JETS. The ink supply system described in U.S. Pat. No. 4,149,172 can only be operated in an upright position and is therefore not practical for use in small-sized portable printers such as electronic calculators, for example, where printing may occur in positions other than an upright position.
It is also advantageous for the ink supply system to have a means for detecting when the ink supply has been exhausted so that it can be replaced or refilled. A device which detects the amount of ink in the ink supply container or bag of an ink-operated printer is described in U.S. Pat. No. 4,202,267 entitled DEVICE FOR MONITORING THE INK SUPPLY IN INK-OPERATED PRINTERS. The device described in U.S. Pat. No. 4,202,267 merely detects when the ink supply is low and does not detect the presence of air bubbles in the ink flow line. Moreover, this device cannot be used in portable printers such as portable hand-held calculators since the device must be operated in an upright position. Accordingly, an ink supply system for an ink jet printer which prevents air bubbles from interrupting the printing operation and which can be incorporated into portable printing devices such as hand-held calculators, is desired.